Keying circuit



Nov. 26, 1946.

J. P. SMITH KEYING CIRCUI T Firled JAanQ 22, 1943 N/ i. .if Mw WM a l/ f wf c 00 w@ wm W. M/ n@ a 6,6/0 VaL M65 aF M552' nventot ACttorneg Patented Nov. 26, 1946 KEYING cmcm'r' John P. Smith, Cranbury, N. J., assigner to Radio Corporation of America, a corporation of Dela- Ware Application January 22, 1943, Serial No. 473,259 s anims. (c1. 25o-27) My invention relates to keying or switching circuitsV and particularly to an electrical circuit wherein electrical pulses are produced under the control of mechanical switching.

One object of the invention is to provide an improved means for obtaining short electrical pulses in response to the actuation of a mechanical switch.

Another object of the invention is to provide an improved means for obtaining electrical pulses which occur synchronously with the rotation or other motion of a mechanical element.

In a preferred embodiment of the invention,

I utilize an asymmetrical multivibrator and connect the mechanical switch in the cathode circuit of the multivibrator tube that has the longer grid capacitor-grid leak resistor time constant. A short electrical pulse is generated in response to closing the cathode circuit by the switch, the pulse ending before the switch is opened. Also, the longer time constant is adjusted so that the grid capacitor of said tube is substantially fully discharged before the switch is again closed.

'I'he invention will be better understood from the following description taken in connection with the accompanying drawing in which Figure 1 is a circuit diagram illustrating one embodiment of the invention,

Figure 2 is a graph that is referred to in explaining the operation of Fig. 1,

Figure 3 is a graph showing the time relations between the switch operation and the multivibrator output, and

Figure 4 is a graph illustrating another method of practicing the invention.

In Fig. l, one embodiment of the invention is shown applied to a system wherein a directional antenna I is rotated about a vertical shaft or axis I I at a constant speed. It is desired to produce a, short electrical pulse each time the antenna I0 reaches a certain position, for example, each time the antenna radiation pattern is pointing north.

A switch i3 located near the antenna Il) is closed periodically by a cam I4 'as the antenna is rotated. The cam I4 may be mechanically coupled to the shaft II as indicated by the broken line I6 sc that the switch I3 closes once for each rotation of the antenna.

A multivibrator I8, which may be located at a point remote from the antenna Ill, comprises two three-element vacuum tubes I9 and 2|. Plate voltage is applied to the tubes through plate resistors 22 and 23. The resistor 24 and capacitor 26 are lter elements.

Tube IS has a grid resistor RI and a grid ca-v pacitor Ci having comparatively large resistance and/or capacity, respectively, whereby the time constant of the circuit RI-CI is comparatively large. Thetube ZI has a grid leak resistor R2 and a grid capacitor C2 having comparatively small values so-that the time constant of circuit R2-C2 is comparatively small. The cathode end of resistor R2 may be connected to a variable tap 28 on a source of positive biasing voltage for controlling the width of the multivibrator pulses. A bypass capacitor 29 may be provided.

The cathode circuit of the tube I9 includes the switch i3 so that the cathode of tube I9 is connected to ground through the conductors 3l and 32 and the switch I3 when the switch I3 is closed. When switch I3 is closed, plate current flows through tube i9 to cause a voltage pulse 35 of short duration to appear on the plate of tube 2I. The action of the circuit in producing the desired short pulse is illustrated by the graph in Fig. 2 showing the voltage on the grid of tube 2l and by the diagram in Fig. 3 showing the different time relations. The action is substantially as follows: When the switch I3 closes there is an increase of current flow through the tube I9 which causes its plate to go more negative and drive the grid of tube 2| more negative. This decreases the plate current through tube ZI to make its plate more positive whereby the grid of tube I 9 is made more positive, thus further increasing the current flow in tube I9 and reinforcing the negative charge on grid capacitor C2. As a result, the tube ZI is almost instantly driven to plate current cutoff to produce the front edge of the short pulse 3S. This cutoi condition is held until the charge of the capacitor C2 leaks oif through resistor R2 enough to let the grid of tube 2! go positive. The plate of tube ZI now goes negative, whereby the capacitor CI is charged to block the tube I9, thus producing the back edge of the said short pulse 36. Because of the comparatively long time constant of the circuit Ci--Ri, the tube I9 remains blocked for a comparatively long period and as a result the interval between the successive short pulses is long compared to the width or duration of these pulses.

The width of the pulses 3S may be controlled by adjusting the tap 28 to change the positive bias on the grid of tube ZI, this bias adjustment having the same effect as changing the values of the elements C2 and R2. In one specific embodiment of the invention the circuit time constants and the bias for tube 2| were so adjusted that the short pulse was approximately 1/20 seconds long while the time for complete discharge of the capacitor CI was three and one-half secends, this being the condition illustrated in Figs. 2 and 3.

As indicated in Fig. 3, the capacitor CI issubstantially completely discharged between the time the antenna switch I3 opens and the time it is again closed. In the example illustrated the switch I3 has an on time of one second and a recurrent closing rate of six seconds.

If desired, the invention may be utilized to provide more than one short pulsefor each actua-` tion of the mechanical switch. In this case the Widths of the short pulses would be determined by the time constant of the circuit C2-R2 and the intervals between pulses would be determined by the time constant of the circuit C I-RI Thus, if instead of the arrangement illustrated in Fig. 3, the switch I3 remains closed for seven seconds, as indicated in Fig. 4, three short pulses would be Ygenerated with an accuracy of timing depending on the multivibrator constants. By reducing the time constant of the circuit RI-CI the natural vperiod of the multivibrator may be decreased so `that for seven seconds on time for switch I3 brator comprising a pairof tubes each having a cathode circuit, an anode vand a control grid and further comprising a grid capacitor and a grid leak resistor connected to the control grids of said tubes, respectively, one of Said tubes having a grid capacitor and a grid leak resistor that have a substantially longer time constant than thel grid capacitor and grid leak resistor of the other tube, a switch connected in the cathode circuit of said one tube, and means for periodically closing said switch for a length of time that exceeds the duration of the shorter duration voltage pulses produced at the plate of said other tube.

3. In combination, an asymmetrical multivibrator comprising a pair of tubes each having a cathode circuit, an anode and a control grid and further comprising a grid capacitor and a grid leak resistor connected to the control grids of said tubes, respectively, one of said tubes having a grid capacitor and a grid leak resistor that have a substantially longer time constant than the grid capacitor and grid leak resistor of the other tube, a switch connected in the cathode circuit of said one tube, and means for periodically closing said switch for a length of time that exceeds the duration of the shorter duration voltage pulses produced at the plate of said other tube but which is less than the natural period of the multivibrator.

JOI-IN P. SMITH. 

